The present invention relates to an electroslag remelting or refining process and more particularly, a method and arrangement for enhancing impurity removal for such process.
A method for the production of ingots from high-melting metals, particularly steel, is already known as described in U.S. Pat. No. 3,571,475 to Holzgruber et al, issued Mar. 16, 1971. In that disclosure, fusion electrolysis, during the melting of at least one self-consuming electrode by alternating current in an electrically conductive liquid slag (the so-called electroslag remelting or refining method), is used to control the direction of metallurgical reactions and to remove undesired elements like sulfur, oxygen, etc. from the melt. The principle utilized there is that the slag, which is present in the molten state in the remelting system, is ionized to a great extent due to the joulean heat generated during the passage of the current. Further, the individual ions are moved in the slag by the superposition of a direct current.
In electroslag remelting or refining with alternating current, the polarity of the consumable electrode and of the liquid slag changes periodically according to the polarity of the a.c. source. When the electrode pases, for example, through the positive half wave, the ingot sump forms the negative pole. By means of at least one non-melting auxiliary electrode, which consists preferably of graphite, a d.c. component is introduced into the remelting system by way of rectifiers so that both the electrode and the ingot have a positive (or negative) potential difference relative to the auxiliary electrode. In this manner, it is possible to start a fusion electrolysis which results in ionic migration to the auxiliary electrode, on the other hand, and to the self-consuming electrode and to the sump, on the other. Depending on the polarity of the auxiliary electrode, certain ions will discharge on the auxiliary electrode and be deposited after reactions with the electrode graphite or atmospheric oxygen. This enhances reactions particularly between the steel and slag phase.
According to this known method it is possible, for example, to increase the desulfurization of steel, where the following reactions take place: EQU (S.sup.--) .fwdarw. S +2 e EQU S + S .fwdarw. S.sub.2 EQU s + 2 o .fwdarw. {so.sub.2 }
the velocity of these reactions is higher with higher migration velocity of the ion. The lower limit of the electromotive force Uo moving the ions can be calculated from the quantities. .DELTA.H, T, .DELTA.S and n, which are typical of the above described reactions. .DELTA.H is the amount of heat in cal/mol used up or released in the course of such a reaction. .DELTA.S is the entropy change in the course of a chemical reaction in cal/.degree.C. Mol. T is the absolute temperature in .degree.K. n is the number of reacted elementary charges per molecule or ion.
The term .DELTA.Ho - T.multidot..DELTA.S = .DELTA.Go is the free standard reaction enthalpy and denotes the energy that is expanded in the course of a chemical reaction.
F is the Faraday constant which indicates the amount of current that is necessary to deposit 1/n mole (=1 val). EQU F = 96,500 (coulomb) = 26.6 Ampere-hours-(Ah)
The d.c. voltage U.sub.G required for the electrolytic treatment of the slag in electroslag-remelting results generally from the equation EQU U.sub.G = U +J.multidot.R
where
U is the theoretical electromotive force (emf) required to activate the chemical process and is calculated from EQU .DELTA. G = .DELTA.Go + R.multidot.T.multidot. 1nK = - n.multidot.F.multidot.U PA1 .rho. is the specific resistance of the slag bath in ohm-centimeters. PA1 1 is the distance of the electrodes in the slag in centimeters. PA1 f is the effective electrode surface in square centimeters. PA1 K is the characteristic from the law of mass action for the respective reaction. PA1 n is the charge number. PA1 U.sub.o is the basic emf (V).
from which we obtain EQU U = R.multidot.T/n F .multidot. 1nK = .DELTA.Go/n.multidot.F
furthermore we have EQU U.sub.o = .DELTA.Go/n.multidot.F
and EQU R = U.sub.G /J = .rho..multidot.1/f ohm
where
As mentioned above, the auxiliary electrode has previously been used only as an anode (or cathode), while the melting electrode or the phase boundary slag-sump served as the opposing poles or (antipoles). Reactions could therefore practically only be started on the non-consuming auxiliary electrode, since the current density was too low at the antipoles, and if reactions took place, tthe deposited reaction products were again introduced into the remelting system by the melting of the electrode.
While in the presently used methods, the undesired components like S.sup.2-, O.sup.2-, N.sup.-, OH.sup.- could be deposited with the auxiliary electrode poled as an anode (which led to an improvement of the desulfurization, prevention of Si- oxidation and decomposition of nitrogen), an effective decomposition of hydrogen in the form of H.sup.+ can only be achieved with the auxiliary electrode poled as a cathode.